JP2011140149A - Method for relieving pressure for injection molding machine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To step up energy consumption saving and realize rapid pressure drop in addition to the materialization of highly precise and stable operating control. <P>SOLUTION: This method for relieving pressure for an injection molding machine involves the following procedures. First, a variable discharge-type hydraulic pump 3 is installed which can control the discharge pressure of at least a hydraulic fluid by variably controlling the number of revolution of a drive motor 2. Next, a specified operating process is controlled in a molding cycle by driving a specified hydraulic actuator (4a, ...) with the help of the variable discharge-type hydraulic pump 3. In this case, the raised mounting residual pressure Pr of the hydraulic circuit 5 is lowered down to the level of a specified target pressure Pn intended for relieving pressure. Yet in this occasion, however, the drive motor 2 is left running free, and the hydraulic circuit 5 and an oil tank 6 in which the residual pressure Pr remains, are set as connected together through the variable discharge-type hydraulic pump 3. Consequently, it is possible to accumulate regenerated electric power Wp to be generated by the drive motor 2 and feed the power including the accumulated regenerated electric power Wp to the drive motor 2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a pressure removing method for an injection molding machine that is suitable for use in releasing the residual pressure of a hydraulic circuit that has risen due to operation of a hydraulic pump.

  In general, in a hydraulic injection molding machine, a predetermined hydraulic actuator is driven by a hydraulic drive unit including a hydraulic pump, and based on this, control is performed for each operation process such as a metering process and an injection process in a molding cycle. In addition, when an operation process is switched, a hydraulic circuit (hydraulic actuator) is usually used so that the behavior (residual pressure) of the previous operation process can be switched smoothly and quickly without affecting the next operation process. The depressurization process for is performed.

  Conventionally, as such a decompression method, a control method for an injection molding machine disclosed in Patent Document 1 already proposed by the present applicant is known. This control method is a control method for variably controlling the number of rotations of a drive motor in a hydraulic pump and controlling a predetermined operation process in a molding cycle by driving a predetermined hydraulic actuator. When the pressure is lowered to the pressure, the drive motor is reversely rotated to forcibly reduce the pressure.

JP 2007-69502 A

  However, the above-described conventional injection molding machine control method (pressure reduction method) has the following problems to be solved.

  First, when reducing the pressure in the operation process to a predetermined pressure, the pressure is forcibly reduced by controlling the reverse rotation of the drive motor so that it is directly related to molding. In other words, the hydraulic pump (drive motor) is actuated in the non-operating section, and so to speak, energy consumption that is essentially unnecessary is generated. Therefore, it is not desirable from the viewpoint of energy saving.

  Second, because the pressure is forcibly reduced by reverse rotation control of the drive motor, the depressurization process itself is faster than, for example, a depressurization method that naturally drops the residual pressure via the pressure control valve. It can be carried out. However, since the rotation of the drive motor is controlled reversely, it takes a certain amount of time until a substantial pressure reduction process is performed, that is, until it can be forcibly reduced. In some cases, it was not necessarily superior. Further, since it is forcibly reduced, it is not desirable from the viewpoint of durability, for example, a load is applied to the drive motor and rubber piping.

  An object of the present invention is to provide a pressure reducing method for an injection molding machine that solves the problems existing in the background art.

  In order to solve the above-described problems, the pressure reducing method for an injection molding machine according to the present invention includes a variable discharge hydraulic pump 3 that can control at least the discharge pressure of hydraulic fluid by variably controlling the rotation speed of the drive motor 2. The residual pressure Pr of the hydraulic circuit 5 that has risen when a predetermined hydraulic actuator (4a ...) is driven by the variable discharge hydraulic pump 3 to control a predetermined operation process in the molding cycle is removed by a predetermined pressure. When the pressure is lowered to the target pressure Pn, the drive motor 2 is brought into a freely rotating state, the hydraulic oil in the hydraulic circuit 5 in which the residual pressure Pr exists is caused to flow back to the oil tank 6 through the variable discharge hydraulic pump 3, and the drive motor 2 The regenerative power Wp generated from the power is stored, and the power including the stored regenerative power Wp is supplied to the drive motor 2.

  In this case, according to a preferred aspect of the invention, an unload pressure can be applied to the predetermined pressure reduction target pressure Pn. Further, the depressurization process can be performed under the condition that the pressure difference Pr−Pn between the residual pressure Pr and the depressurization target pressure Pn is equal to or greater than a preset value Ps that is a predetermined magnitude set in advance. On the other hand, this depressurization process can be performed in a switching section from any operation process to the next operation process, and this switching section can include at least a switching section from the pressure holding process to the measurement process. . On the other hand, the variable discharge hydraulic pump 3 is provided with a setting function for setting a plurality of fixed discharge flow rates Qm and Qs having different maximum discharge flow rates, and can be switched to the largest fixed discharge flow rate Qm at the time of pressure release processing.

  According to the pressure reducing method of the injection molding machine according to the present invention by such a method, the following remarkable effects are obtained.

  (1) In addition to the effects of avoiding problems such as the generation of shock pressure at the time of operation switching, enabling smooth switching and realizing highly accurate and stable operation control, basically, the drive motor 2 Since it can be realized by setting the rotation free state, unnecessary energy consumption associated with the decompression can be avoided, and the energy saving can be further improved by reusing the regenerative power Wp generated from the drive motor 2. . In particular, in the case of a hydraulic injection molding machine that uses a hydraulic pump as a driving source, a plurality of processes are continuously performed by a single driving source, so that each process is individually performed by a plurality of driving sources. The utilization efficiency of regenerative power can be increased, which is more preferable from the viewpoint of improving energy saving.

  (2) Since it is sufficient to make the drive motor 2 in a freely rotating state, it is possible to promptly shift to a substantial pressure removal process. Therefore, even if the method of pressure reduction by naturally dropping is adopted, the overall required time for pressure reduction is not necessarily disadvantageous, and the advantage that the pressure can be quickly lowered can be secured. Further, since there is no reverse rotation operation of the drive motor 2 or the like, there is no problem that an unnecessary load is applied to the drive motor 2 or the rubber pipe, which is advantageous from the viewpoint of durability.

  (3) If the unload pressure is applied to the predetermined target pressure Pn to be removed according to the preferred embodiment, the substantial pressure removal with respect to the residual pressure Pr in the entire hydraulic circuit 5 including the hydraulic actuator such as the injection cylinder can be ensured. Can be done.

  (4) According to a preferred aspect, the depressurization process is performed on condition that the pressure difference Pr-Pn between the residual pressure Pr and the depressurization target pressure Pn is equal to or greater than a preset value Ps that is a predetermined magnitude. In this case, if the residual pressure Pr does not need to be depressurized, the depressurization process is not performed. Therefore, the useless depressurization process is avoided and the process immediately proceeds to the next operation process. This can contribute to shortening the molding cycle and improving production efficiency.

  (5) According to a preferred embodiment, the depressurization process is performed in a switching section from any operation process to the next operation process, and in particular, at least a switching section from the pressure holding process to the weighing process can be included as the switching section. For example, since the depressurization process is realized in the switching section of the operation process having the greatest influence at the time of injection molding, it is possible to further improve the measurement accuracy of the measurement process and stabilize the measurement.

  (6) According to a preferred embodiment, the variable discharge hydraulic pump 3 is provided with a setting function for setting a plurality of fixed discharge flow rates Qm and Qs having different maximum discharge flow rates, and the maximum fixed discharge flow rate Qm is set during the depressurization process. By switching, it is possible to perform the depressurization process more quickly and smoothly (reliably).

The flowchart for explaining step by step the pressure release method of the injection molding machine according to a preferred embodiment of the present invention, The block diagram including the hydraulic drive part of the injection molding machine used for the same decompression method, Block circuit diagram of hydraulic drive unit in the same injection molding machine, A list showing the specific operation names and unloading commands that are performed in the weighing process and injection process, which are part of the operation process in the molding cycle of the injection molding machine, Hydraulic circuit diagram extracted hydraulic circuit on the injection device side of the injection molding machine, Change diagram of pump pressure and drive motor speed with respect to time when using the same decompression method,

  Next, preferred embodiments according to the present invention will be given and described in detail with reference to the drawings.

  First, the configuration of an injection molding machine M that can perform the pressure-removing method according to the present embodiment will be described with reference to FIGS.

  In FIG. 2, M is an injection molding machine, and includes an injection device Mi and a mold clamping device Mc. The injection molding machine M includes, as a hydraulic actuator (4a...), An injection cylinder 4a for moving the screw 13 built in the heating cylinder 12 in the injection apparatus Mi, and a metering motor (oil motor) 4b for rotating the screw 13. The mold clamping device Mc includes a mold clamping cylinder 4c that performs mold opening / closing and mold clamping with respect to the mold 15 and a projecting cylinder 4d (FIG. 3) that ejects a molded product from the mold 15 (ejector). In addition, an injection device moving cylinder 4e (FIG. 3) is provided to move the injection device Mi forward and backward to perform nozzle touch on the mold 15 or release thereof.

  On the other hand, reference numeral 21 denotes a hydraulic drive unit, which includes a variable discharge hydraulic pump 3 and a switching valve circuit 22 serving as a hydraulic drive source. The variable discharge hydraulic pump 3 includes a pump unit 25 and a servo motor 2s (drive motor 2) that rotationally drives the pump unit 25. An AC servo motor is used as the servo motor 2 s, and this servo motor 2 s is connected to a power source (three-phase AC power source) 7 through a regenerative circuit 9 and a converter 8. The regenerative circuit 9 includes a smooth power storage unit 9a and an inverter (servo circuit) 9b from the power source 7 side, and connects the servo motor 2s to the inverter 9b. In this case, the regenerative circuit 9 itself can use a known circuit configuration. Further, the servo motor 2s is provided with a rotary encoder 2se for detecting the rotation speed of the servo motor 2s, and the rotary encoder 2se is connected to the inverter 9b.

  Moreover, the pump part 25 incorporates the pump body 26 comprised with a swash plate type piston pump. Therefore, the pump unit 25 includes the swash plate 27. If the swash plate angle Rs, which is the inclination angle of the swash plate 27, is increased, the stroke of the pump piston in the pump body 26 increases, the discharge flow rate increases, and the swash plate angle Rs increases. If the plate angle Rs is reduced, the stroke of the pump piston is reduced and the discharge flow rate is reduced. Therefore, by setting the swash plate angle Rs to a predetermined angle, a fixed discharge flow rate Qm... At which the maximum discharge flow rate is fixed to a predetermined size can be set. In the example, two fixed discharge flow rates, that is, a fixed discharge flow rate Qs having a small flow rate and a fixed discharge flow rate Qm having a large flow rate can be set. The small fixed discharge flow rate Qs can be set to a standard discharge flow rate, and the large fixed discharge flow rate Qm can be set to about twice the small fixed flow rate Qs. In particular, the large fixed discharge flow rate Qm can be set in consideration of a size that does not adversely affect the drive motor 2 (servo motor 2s) for a short time. Further, a control cylinder 28 and a return spring 29 are attached to the swash plate 27, and the control cylinder 28 is connected to a discharge port of the pump unit 25 (pump machine body 26) via a switching valve (electromagnetic valve) 30. Thus, the angle of the swash plate 27 can be changed by the control cylinder 28.

  Therefore, with such a configuration, the discharge flow rate and discharge pressure of the hydraulic pump 3 can be varied by variably controlling the rotation speed of the servo motor 2s. Therefore, each operation process in the molding cycle can be controlled by the drive control of the cylinders 4a, 4c, 4d, 4e and the metering motor 4b described above. In addition, 31 is a pressure sensor which detects the discharge pressure of the hydraulic pump 3, and can detect the residual pressure Pr used in the decompression method according to the present embodiment.

  On the other hand, the suction port of the pump unit 25 is connected to the oil tank 6, the discharge port of the pump unit 25 is connected to the primary side of the switching valve circuit 22, and the secondary side of the switching valve circuit 22 is As shown in FIG. 3, the injection cylinder 4a, the metering motor 4b, the clamping cylinder 4c, the protruding cylinder 4d, and the injection device moving cylinder 4e constituting the hydraulic actuator in the injection molding machine M are connected. Therefore, the switching valve circuit 22 includes at least switching valves (electromagnetic valves) 22a, 22b, 22c, and 22d connected to the injection cylinder 4a, the metering motor 4b, the clamping cylinder 4c, the protruding cylinder 4d, and the injection device moving cylinder 4e, respectively. And 22e. Each switching valve 22a is composed of one or two or more valve parts and necessary accessory hydraulic parts, and includes at least an injection cylinder 4a, a metering motor 4b, a mold clamping cylinder 4c, a protruding cylinder 4d, and an injection. It has a switching function related to supply, stop, and discharge of hydraulic oil to the device moving cylinder 4e. The switching valve circuit 22 and each hydraulic actuator (4a...) Constitute a hydraulic circuit 5.

Reference numeral 41 denotes a molding machine controller. The molding machine controller 41 is connected to the inverter 9b and the converter 8 described above, and to each switching valve 22a, 22b, 22c, 22d, 22e using the electromagnetic valve, the switching valve 30, and the pressure sensor 31. To do. The molding machine controller 41 has a computer function including a CPU, a memory, a power supply unit, and the like. The molding machine controller 41 controls the entire system and includes a processing program (sequence program) for executing the decompression method according to the present embodiment. .
Further, FIG. 4 shows an extracted hydraulic circuit 51 (5) on the injection device Mi side. In the figure, reference numerals 52 and 53 denote switching valves as valve parts, 54 denotes a back pressure control circuit, and each part and circuit are connected or piped as shown in the figure. 4ap indicates a single rod type piston built in the injection cylinder 4a, and 4ar indicates a rear oil chamber of the injection cylinder 4a. 2 and 3 in the other configuration of FIG. 4 are assigned the same reference numerals to clarify the configuration, and detailed description thereof will be omitted, and the operation (function) of the hydraulic circuit 51 will be omitted. Will be described later.

  Next, a pressure removal method according to the present embodiment using such an injection molding machine M will be described with reference to FIGS.

  FIG. 5 is a list showing specific operation names and presence / absence of an unload command performed in the weighing process and the injection process, which are a part of the operation processes in the molding cycle of the injection molding machine M. In this case, in the weighing process, an “injection decompression” process, a “decompression before weighing” process, a “measurement start timing adjustment” process, a “weighing 1st speed” process, a “weighing 2nd speed” process, a “weighing 3rd speed” process , “Measurement stop timing adjustment” process, “decompression start timing adjustment” process, “decompression” process are performed sequentially, especially “injection depressurization” process that performs depressurization (pressure release) after the injection process is completed, and weighing “Weighing start timing adjustment” process for adjusting timing when starting measurement, “Weighing stop timing adjustment” process for adjusting timing when stopping weighing, “Decompression start timing” for adjusting timing when starting decompression process In the “adjustment” process, a process for reducing the pressure to the unload pressure (removal target pressure) Pn is performed, and thereafter, the switching control of the corresponding switching valve is performed.

  Similarly, in the injection process, “injection start timing adjustment” process, “injection switching valve ON” process, “injection 1st speed” process, “injection 2nd speed” process, “injection 3rd speed” process, “injection 4th speed” ”,“ Injection 5th speed ”process,“ Injection 6th speed ”process,“ Holding pressure 1 pressure ”process,“ Holding pressure 2 pressure ”process,“ Holding pressure 3 pressure ”process, In the “injection start timing adjustment” process for adjusting the timing when starting injection after the end, and the “injection switch valve ON” process for switching the injection switching valve 53 to the ON (open) side when starting injection, In either case, a process for reducing the pressure to the unload pressure (decompression target pressure) Pn is performed, and thereafter, the switching control of the corresponding switching valve is performed.

  The unload pressure Pn is not a zero pressure but a no-load pressure that is about 6% with respect to the maximum pressure (rated pressure). In this way, if the unload pressure Pn is applied to the target pressure for pressure removal, substantial pressure reduction with respect to the residual pressure Pr in the entire hydraulic circuit 5 including the hydraulic actuator such as the injection cylinder 4a can be reliably performed. it can.

  Next, a specific processing procedure of the decompression method will be described with reference to FIG. 1 and FIG. 4 by taking an “injection decompression” process in which the decompression is performed immediately after the injection process is completed as an example. FIG. 1 is a flowchart showing a processing procedure when performing the “injection decompression” processing.

  Assume that the injection process is completed (steps S1 and S2). In the injection process, the switching valve 52 is switched to the symbol b and the switching valve 53 is switched to the symbol a in FIG. 4. When the injection process is completed, the “holding pressure” of the injection process shown in FIG. The “3 pressure” process has been completed.

  Upon completion of the injection process (“holding pressure 3 pressure” process), first, the molding machine controller 41 determines that the pressure difference Pr−Pn between the residual pressure Pr detected by the pressure sensor 31 and the unload pressure Pn is a predetermined value set in advance. It is determined whether or not the value is equal to or larger than a set value Ps that is a magnitude of (step S3). In this case, the set value Ps is a threshold value for determining whether or not the magnitude of the residual pressure Pr needs to be decompressed. Therefore, the depressurization process is performed on the condition that the pressure difference Pr−Pn is equal to or larger than the set value Ps. As a result, if the residual pressure Pr does not need to be depressurized, the depressurization process is not performed, so that unnecessary depressurization process is avoided and the process immediately proceeds to the next operation process. This can contribute to shortening the molding cycle and improving production efficiency.

  On the other hand, if the pressure difference Pr-Pn between the residual pressure Pr and the unload pressure Pn is greater than or equal to the set value Ps, the molding machine controller 41 outputs an unload command (step S4). Thereby, the power supply to the servo motor 2s is stopped, and the servo motor 2s is in a freely rotating state (step S5). As described above, it is sufficient to stop the power supply to the servo motor 2s by the unload command, and the reverse rotation control or the valve switching process for the servo motor 2s is not necessary for the transition to the pressure removal process. In addition, when the hydraulic pump 3 which can set several fixed discharge flow rate Qm ... is provided, it switches to the largest fixed discharge flow rate Qm. Thereby, the depressurization process can be performed more quickly and smoothly (reliably). At the time of the depressurization process, the hydraulic oil of the hydraulic circuit 5 including the rear oil chamber 4ar of the injection cylinder 4a in which the residual pressure Pr exists is supplied to the oil tank 6 via the hydraulic pump 3 by the residual pressure Pr at the time when the injection process is completed. (Step S6).

  As a result, the servo motor 2s of the hydraulic pump 3 rotates in the reverse direction (passive rotation), and the servo motor 2s functions as a generator to generate regenerative power Wp (step S7). The regenerative power Wp is regenerated by the regenerative circuit 9. That is, the generated regenerative power Wp (AC power) is applied to the inverter 9b from the reverse direction, and is converted to DC power by a built-in conversion function (step S8). Further, the DC power obtained from the inverter 9b is applied to the smooth power storage unit 9a having a capacitor, and is smoothed and stored (step S9).

  On the other hand, the residual pressure Pr is detected by the pressure sensor 31 and the detection result is given to the molding machine controller 41. Then, the molding machine controller 41 monitors whether or not the residual pressure Pr has reached the unload pressure Pn. If the residual pressure Pr has reached the unload pressure Pn, the decompression process is terminated (step S10). When the decompression process is completed or the decompression process is not performed, the metering process is performed by switching to the metering process by switching the switching valve 53 to the symbol b in this state (step S11). At this time, since the hydraulic pump 3 and the hydraulic circuit 5 are equal to or lower than the unload pressure Pn, problems such as the generation of shock pressure at the time of valve switching (operation switching) can be avoided and smooth switching can be performed. In addition, highly accurate and stable operation control can be performed in the weighing process.

  In the weighing process, the hydraulic pump 3 is operated by the operation of the servo motor 2s. In this case, three-phase AC power is applied from the power source 7 to the converter 8 and converted into DC power. This DC power is applied to the smoothing power storage unit 9a, smoothed, and added to the regenerative power Wp stored immediately before. Then, the DC power including the regenerative power Wp is converted into AC power by being applied to the inverter 9b and supplied to the servo motor 2s. At this time, a control signal from the molding machine controller 41 is given to the inverter 9b, and the rotation speed of the servo motor 2s is variably controlled. That is, the discharge pressure and the discharge flow rate of the hydraulic pump 3 are inverter-controlled. Thereafter, when the weighing process is completed, the process proceeds to the injection process (steps S12 and S13).

  FIG. 6 shows test data relating to changes in the residual pressure Pr and the rotational speed Rr of the servo motor 2s with respect to time [seconds] when using the decompression method according to the present embodiment (the method of the present invention). This is shown in contrast to the first control method, that is, a method for forcibly reducing the pressure by controlling the servo motor to rotate backward (conventional method). In the figure, Pri is the change in the residual pressure Pr when using the method of the present invention, Prr is the change in the residual pressure Pr when using the conventional method, and Rri is the rotation of the servo motor 2s when using the method of the present invention. A change in the number Rr and Rrr indicate a change in the rotational speed Rr of the servo motor 2s when the conventional method is used. As is clear from the figure, in the case of the method of the present invention, the time to decrease to the unload pressure Pn is approximately 0.2 to 0.3 [seconds] slower than the case of the conventional method. The delay remains almost unaffected.

  In the above, taking the “injection depressurization” process in the weighing process as an example, the case where the depressurization process is performed in the switching section from the holding pressure process to the weighing process has been shown. Switching from any operation process to the next operation process has been shown. In the section, the same decompression process can be performed. Specifically, in the “weighing start timing adjustment” process, the “measurement stop timing adjustment” process, the “decompression start timing adjustment” process, the “injection start timing adjustment” process, and the “injection switching valve ON” process in FIG. The same control, that is, the unload command Pn is output, and the same pressure removal process can be performed. FIG. 5 shows only the weighing process and the injection process, but the same control is performed in other operation processes in the molding cycle, for example, a mold opening / closing process, a mold clamping process, and a protruding (ejector) process. Can do. In particular, if the switching section includes the switching section from the holding pressure process to the weighing process illustrated in the example, the depressurization process is realized in the switching section of the operation process that has the greatest influence during injection molding. There is an advantage that the measurement accuracy can be further improved and the measurement can be stabilized.

  Therefore, according to such a pressure releasing method of the injection molding machine M according to the present embodiment, it is possible to avoid problems such as generation of shock pressure at the time of operation switching, and to perform smooth switching, with high accuracy and stability. In addition to the effect that the operation control can be realized, basically, it can be realized by setting the servo motor 2s to the rotation free state, so that unnecessary energy consumption due to the decompression can be avoided and the servo motor 2s is generated from the servo motor 2s. By reusing the regenerative power Wp, it is possible to further improve energy saving. In particular, in the case of a hydraulic injection molding machine that uses a hydraulic pump as a driving source, a plurality of processes are continuously performed by a single driving source, so that each process is individually performed by a plurality of driving sources. The utilization efficiency of regenerative power can be increased, which is more preferable from the viewpoint of improving energy saving. Further, since it is sufficient to make the servo motor 2s in a freely rotating state, it is possible to promptly shift to a substantial pressure removal process. Therefore, even if the method of pressure reduction by naturally dropping is adopted, the overall required time for pressure reduction is not necessarily disadvantageous, and the advantage that the pressure can be quickly lowered can be secured. Moreover, since the servo motor 2s is not accompanied by a reverse rotation operation or the like, there is no problem that an unnecessary load is applied to the servo motor 2s or the rubber piping, which is advantageous from the viewpoint of durability.

  The preferred embodiment has been described in detail above, but the present invention is not limited to such an embodiment, and the configuration and method of the details are arbitrarily changed without departing from the gist of the present invention. , Can be added or deleted. For example, although the servo motor 2s is shown as the drive motor 2, other various drive motors can be used. Moreover, although the case where unload pressure was applied as the pressure reduction target pressure Pn was shown, it is not limited to this unload pressure. Furthermore, although the case where it performed on condition that the pressure difference Pr-Pn of the residual pressure Pr and the pressure reduction target pressure Pn is more than the preset value Ps used as the predetermined magnitude | size preset was shown, it is limited to such conditions. It is not something.

  The depressurization method according to the present invention can be used for various injection molding machines that require depressurization with respect to the residual pressure of the hydraulic circuit that has risen due to the operation of the hydraulic pump.

  2: drive motor, 3: variable discharge hydraulic pump, 4a ...: hydraulic actuator, 5: hydraulic circuit, 6: oil tank, Pr: residual pressure, Pn: target pressure for pressure removal, Wp: regenerative power, Ps: set value

Claims (6)

  A variable discharge hydraulic pump capable of controlling at least the hydraulic oil discharge pressure by variably controlling the rotational speed of the drive motor is provided, and a predetermined hydraulic actuator is driven by the variable discharge hydraulic pump to perform a predetermined operation in a molding cycle. In the pressure reducing method of the injection molding machine that reduces the residual pressure of the hydraulic circuit that has risen when the process is controlled to a predetermined pressure target pressure, when the residual pressure is reduced to the predetermined target pressure, The drive motor is in a freely rotating state, the hydraulic oil in the hydraulic circuit in which the residual pressure exists is caused to flow back to the oil tank via the variable discharge hydraulic pump, and regenerative power generated from the drive motor is stored. A method for depressurizing an injection molding machine, comprising supplying electric power including regenerated electric power to the drive motor.   2. The pressure removing method for an injection molding machine according to claim 1, wherein the predetermined pressure removal target pressure is an unload pressure.   3. The injection molding machine according to claim 1, wherein the depressurization process is performed under a condition that a pressure difference between the residual pressure and the depressurization target pressure is equal to or greater than a predetermined size set in advance. Decompression method.   4. The method of depressurizing an injection molding machine according to claim 1, wherein the depressurization process is performed in a switching section from an arbitrary operation process to a next operation process.   5. The pressure reducing method for an injection molding machine according to claim 4, wherein the switching section includes at least a switching section from a pressure holding process to a weighing process.   2. The variable discharge hydraulic pump has a setting function for setting a plurality of fixed discharge flow rates with different maximum discharge flow rates, and switches to the largest fixed discharge flow rate during the depressurization process. Pressure release method for injection molding machines.

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